An Open Shortest Path First (OSPF) protocol is an Interior Gateway Protocol (IGP) for making a routing decision within a single Autonomous System (AS).
A Link-State Advertisement (LSA) is a packet used in a link-state protocol, which includes information on a neighbor and a passage cost. A router uses an LSA for maintaining a route option table. There are eleven types of LSAs in the OSPF, listed as follows.
An LSA 1 is a Router LSA.
An LSA 2 is a network LSA.
An LSA 3 is a Network summary LSA.
An LSA 4 is an AS Border Router (ASBR) summary LSA.
An LSA 5 is an AS external LSA.
An LSA 6 is a Group membership LSA.
An LSA 7 is a Not-So-Stubby Area specific AS external LSA (NSSA External LSA).
An LSA 8 is an external attributes LSA for a Border Gateway Protocol (External attributes LSA for BGP).
An LSA 9 is a link-scoped Opaque LSA.
An LSA 10 is an area-scoped Opaque LSA.
An LSA 11 is an AS-scoped Opaque LSA.
With the existing OSPF protocol, there are four LSAs capable of announcing a 32-bit IP address, i.e., LSA 1, LSA 3, LSA 5, and LSA 7; with both LSA 5 and LSA 7 being AS external LSAs. A 32-bit IP address may be announced as follows with such an LSA.
(1) A 32-bit IP address may be announced with LSA 1 by configuring an interface as a Point-to-Multipoint (PTMP) interface with a 32-bit IP address.
(2) A 32-bit IP address may be announced with LSA 3 by transforming LSA 1 in form of LSA 3 in another area, corresponding to 32-bit routing.
(3) No 32-bit IP address may be announced with LSA 5, LSA 7 in existing art.
In a practical application, 32-bit IP address announcement with LSA 1 may be implemented only with a loopback interface, with the requirement of configuring any interface as a PTMP interface, which is impractical. In 32-bit IP address announcement with LSA 3, as LSA 3 is transformed from LSA 1, accurate correspondence between an address and a node is unavailable to another node.